Electrically heated paving screed

Abstract

A heated screed assembly (10) for use with a paving machine (12) includes a screed (16). An elastomeric, electrically-powered heating element (36) is carried on the upper surface (28) of the screed. A retaining member (48) is disposed to overly the heating element, whereby the heating element is retained in contact with the upper surface of the screed during operation of the paving machine. Electric power is supplied to the elastomeric heating element by a generator (44).

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to asphalt paving equipment, and more particularly to floating screeds used in compacting paving material.

BACKGROUND OF THE INVENTION

The laying of asphalt paving material on road surfaces entails spreading an aggregate-filled tar-based paving material on a prepared road bed. The paving material is spread while hot and is then compacted so that upon cooling a hard pavement surface is formed. Conventional paving machines utilize a heavy metal plate termed a "screed," usually constructed of steel or iron, to compact the paving material. The screed is typically mounted on pivot arms at the rear end of the paving machine, and "floats" up and down in the vertical direction as it is pulled over the paving material. The weight of the screed, as well as other structures carried on the screed, acts to compress and tamp the paving material into a compact layer.

To facilitate compaction of the paving material, the screed must be heated, typically to a temperature of about 180° to 300° F., to assist the paving material in flowing under the screed and to reduce adhesion of the paving material to the screed. If the screed is not adequately heated, the tar in contact with the bottom of the screed begins to harden, resulting in buildup of paving material and excessive drag.

Most conventional screeds are heated through the use of fossil fuel powered burners that heat the upper surface of the screed by the direct application of flame or hot exhaust gases. For example, many conventional screed heaters are powered with propane gas or fuel oil. The use of fossil fuel burners to heat screeds has several drawbacks. Combustion of fossil fuels generates large amounts of smoke, particularly when fuel oils are burned.s essentially the entire end-to-end length of the screed 16, except for a small space between the heating elements and on either end of the heating element, the screed 16 is heated substantially uniformly and evenly across its entire length. Heat from the heating elements 36 also conducts along the width of the screed 16, which is only a relatively short distance, such that substantially the entire bottom surface 30 of the screed 16 is heated to a uniform temperature.

To reduce loss of heat from the beating elements 36, a layer of insulation material 46 is positioned to overly each heating element 36. Two sheets of insulation material 46 are employed, with each sheet covering either the left portion 32 or right portion 34 of the screed. One suitable type of insulation has been found to be R-9 fiberglass insulation, 1/2 thick, including a foil layer that is adhered to the upper surface of the insulation.

The heating elements 36 and insulation 46 are loosely positioned on top of the upper surface 28 of the screed 16. They are not mechanically constrained in their installed positions, except for by the forward face plate portion 24 and tail portion 26 of the screed 16 and the side plates 18. Thus the heating elements 36 are substantially free to move in all directions along the plane defined by the upper surface 28 of the screed 16, with the limits of this movement being defined by the upper face plate portion 24, tail portion 26 and side plates 18.

However, in order to ensure that movement of the heating elements 36 along the plane of the upper surface 28 is substantially prevented during operation of the screed assembly 10, and to also ensure that the heating elements 36 stay in intimate contact with the screed 16 while being vibrated during operation, retaining plate assemblies 48 are positioned atop each sheet of insulation material 46. Each retainer plate assembly 48 thus overlies both a sheet of insulation material 46 and a heating element 36. Each retainer plate assembly 48 is formed from two parallel, spaced elongate plates 50. The distance between, and parallel orientation of, the plates 50 is maintained by a plurality of cross plate members 52 that are welded across the tops of the plates 50.

The length and width of the retainer plate assemblies 48 is nearly the length and the width of the sheets of insulation material 46. Preferably, the retainer plate assemblies 48 are constructed from steel. The weight of the retainer plate assemblies 48 is sufficient to slightly compress the insulation material 46 and heating elements 36. The elastomeric material employed to form the heating elements 36 has a relatively high frictional coefficient, such that heating element 36 does not move substantially during operation in the plane of the screen. The weight of the retaining plate assembly 48 also prevents vertical separation of the heating element 36 from the screen 16.

Because of the shock absorbence provided by the elastomeric body 38 surrounding the resistive conductor 40, and the secure positioning provided by the retaining plate 48, breakage and failure of the resistive conductor 40 is avoided. It should be apparent to those of ordinary skill in the art that other electric heating elements could be used in place of the elastomeric heating element 36, in accordance with the present invention. For example, cal-rods or other electric heating elements could be welded to the upper surface of the screed 16. However, such rigid heating elements are not preferred because of their potential for breakage, and because of the stresses that could be induced by welding or other form of fixed securement to the screed 16. Because the elastomeric heating elements 36 are not fixedly secured, no additional stresses are introduced to the screed 16.

Referring to FIG. 3, the screed 16 is mounted to a mold board 54 and the deck plate 20 in a conventional manner. One such suitable mounting is the use of bolts 58 that pass through apertures 60 and are received within internally threaded apertures, formed along the rearward edge of the screed 16 and within mounting blocks 62 attached to the inside of the forward face plate portion 24 of the screed 16.

The configuration of the heated screed 10 of the present invention allows for rapid heating of the screed 16 to operation temperature. Screeds are conventionally operated at temperatures ranging from 180° to 300° F. A large screed (i.e., extending the full width of a conventional paving machine) can be brought up to an operation temperature of 190° in a time period of 12 minutes by using the heating system of the present invention. This is compared to 40 minutes typically required to bring a screed of the same size to operation temperature when heated with fossil fuels. When a conventional liquid circuit heating system is utilized, a large screed of this size may require in excess of 40 minutes to bring the screed up to operation temperature.

During operation, heat may be applied to the screed 16 either continuously or intermittently, depending on ambient temperatures and operation speeds. For intermittent operation, the supply of power to the heating elements 36 can be either manually controlled through the provision of a switch (not shown), or automatically through the provision of a thermostat (not shown).

While the foregoing heated screed 10 has been described for use with a main screed that extends the entire width of a paving machine, it should be apparent to one of ordinary skills in the art that the present invention can be readily adapted for heating of smaller side-extension screeds, which are used to extend the paving width afforded by a paving machine. This adaptation merely requires scaling down the size of the system components.

While the preferred embodiment of the present invention has been illustrated and described, various modifications, alterations and substitutions can be made by one of ordinary skill in the art, based on the disclosure contained herein, without departing from the spirit and scope of the invention. It is therefore intended that the scope of letters patent granted hereon be limited only by the definitions contained in the appended claims.

Claims

1. A heated screed assembly for use with a paving machine, comprising:

a screed;

an elastomeric, electrically powered heating element assembly disposed in contact with the screed, wherein the heating element assembly overlies and evenly heats a majority of the operative area of the screed; and

an electric power supply.

2. The heated screed of claim 1, wherein the heating element assembly comprises an elastomeric pad in which is embedded a resistive conductor.

3. The heated screed of claim 2, wherein the elastomeric pad is disposed to cover a portion of an upper surface defined by the screed.

4. The heated screed of claim 3, wherein the elastomeric pad is retained in position by the weight of a retainer plate disposed to overlie the pad.

5. The heated screed of claim 4, further comprising a layer of insulation disposed between the elastomeric pad and the retainer plate.

6. The heated screed of claim 1, wherein the electric power supply comprises an electric generator.

7. The heated screed of claim 6, wherein the electric generator is powered by the paving machine.

8. A heated screed assembly for use with a paving machine, comprising:

a screed having a length and a width defining an upper surface;

an electrically powered resistive heating element including means for cushioning the heating element, carried on the upper surface of the screed, wherein the heating element is configured as a flat sheet that evenly heats the screed along the length of the screed; and

a retaining member disposed to overlie the heating element, whereby the heating element is retained in contact with the upper surface of the screed during operation of the paving machine.

9. The heated screed of claim 8, wherein the resistive heating element includes an embedded resistive conductor.

10. The heated screed of claim 9, wherein the body cushioning means comprises an elastomeric sheet.

11. The heated screed of claim 8, wherein movement of the resistive heating element along the upper surface of the screed is substantially prevented by friction between the resistive heating element and the upper surface of the screed induced by the weight of the retaining member.

12. The heated screed of claim 11, wherein the resistive heating element comprises a sheet and the retaining member comprises a retaining plate.

13. The heated screed of claim 12, wherein the retaining plate comprises a grid of plate submembers.

14. The heated screed of claim 8, further comprising a layer of insulation disposed to insulate a first side of the resistive heating element that is opposite of a second side of the resistive heating element that is in contact with the screed.

15. The heated screed of claim 14, wherein the layer of insulation is disposed between the resistive heating element and the retaining member.

16. The heated screed of claim 8, further comprising an electric power supply.